The present invention relates to an electric power steering (EPS) system of an automobile; and, more particularly, to an operational method of an EPS system capable of reducing a yawing moment of the automobile by selectively adjusting steering angles of wheels depending on an operational state of an anti-lock brake system (ABS).
An anti-lock brake system (ABS) operates to increase or decrease hydraulic pressure applied to wheel cylinders of wheels when an automobile brakes. By such operations of the ABS, a cornering force suitable for maintaining an effective steering action and a slip ratio having a largest frictional coefficient capable of minimizing a braking distance for stopping can be obtained.
In general, the ABS comprises wheel speed sensors for detecting rotational speeds of the wheels, each wheel speed sensor being installed at respective front wheels and rear wheels of the automobile, ABS actuators installed at a pipe line between a master cylinder and wheel cylinders so as to adjust a braking power by decreasing or increasing a braking hydraulic pressure within each wheel cylinder, and an ABS electronic control unit (ECU) for controlling the ABS actuators and the wheel speed sensors.
First, the ABS ECU calculates a speed of the automobile by using a rotational speed of each wheel generated by each wheel speed sensor and simultaneously detects a rotational state of each wheel. Then, the ABS ECU commands the ABS actuator to increase or decrease the braking hydraulic pressure to obtain an optimum braking power suitable for a road surface condition. In other words, when a frictional force between a left tire and a road surface is different from that between a right tire and the road surface (split-xcexc), the ABS ECU controls the braking hydraulic pressure to prevent a risk that might occur due to a lock phenomenon of the wheels. FIG. 3A shows an automobile which is revolved due to a yawing moment generated by a difference in the road frictional forces of the left and the right tires.
When a driver suddenly brakes an automobile on a slippery road, the driver tends to hastily steer the automobile for the purpose of preventing a slippage thereof. Accordingly, the automobile is often over-steered, thereby causing a danger for the driver.
As a solution to this problem, a steering effort required to steer the automobile is increased in a conventional EPS system to make a steering response smaller than that of the normal case. The EPS system is widely used to control the current of the steering motor such that the steering effort for obtaining a specific steering result may be attained. Thus, the EPS system makes it possible that a driver can turn a vehicle by operating a steering wheel with a small steering effort, wherein the steering wheel can be operated with a small steering effort when the vehicle is idling or travels at a low speed, whereas a larger steering effort is required when the vehicle travels at a relatively high speed.
However, when road frictional coefficients of the left and the right tires are different from each other in the conventional EPS system, even a larger steering effort won""t remove a risk of accident. Moreover, if an inexpert driver confusedly misses a steering wheel, it is highly likely that an automobile may be slid away from a road to cause an accident.
It is, therefore, an object of the present invention to provide a method for operating an automobile EPS system capable of reducing a yawing moment of an automobile by automatically adjusting a steering angle according to a braking state of the automobile and thus allowing the automobile to move linearly even when the automobile suddenly brakes on a road surface where a left and a right tire of the automobile have different road frictional coefficient.
In accordance with the present invention, there is provided a method for operating an automobile electric power steering (EPS) system capable of selectively adjusting a steering angle depending on a braking state of an automobile, comprising the steps of:
making a decision about whether or not the automatic adjustment of the steering angle is performed based on a speed of the automobile due to the braking state and a rotational state of each wheel;
setting a cooperative steering angle to cooperate the braking state based on the speed of the automobile and decision about whether or not the automatic adjustment of the steering angle is performed;
determining a target steering angle for the automatic adjustment based on the cooperative steering angle and a manual steering angle of a steering wheel; and
performing a feedback control on the cooperative steering angle such that the steering angle of said each wheel becomes close to the determined target steering angle.